1. Field of the Invention
The present invention relates in general to an over-voltage protection circuit (also called a limiter), and more specifically, to an over-voltage protection circuit having a minimum amount of leakage current when an input voltage is below a reference voltage, and having a highly increased leakage current when the input voltage is above the reference voltage.
2. Description of the Related Art
RFID is the abbreviation for Radio Frequency Identification, which also is called a smart tag. RFID is a non-contact recognition system that uses small chips attached to all kinds of items to transfer and process data on objects and their surroundings via radio frequencies.
The basic elements of an RFID system include an antenna, a tag and a reader. The reader identifies a thin planar tag attached to an object, and processes data. Among these RFID systems, a low-frequency identification system (30 kHz-500 kHz) is used in short-distance (≦1.8 m) data communication, whereas a high-frequency identification system (850 MHz-950 MHz, or 2.4 GHz-2.5 GHz) can transfer data from a long distance (greater than 27 m).
An RF tag is composed of a semiconductor transponder chip and an antenna, and there are two types of RF tags: passive tags and active tags. Passive tags require no internal power source because they receive energy from a reader's radio frequency signal, whereas active tags require a built-in power source (e.g., a battery) for operation.
To obtain data from a passive RF tag, a reader supplies power to the tag through the tag's antenna. Then, the RF tag transmits data as a response to receiving the power. Usually, power is supplied from the antenna based on two methods: one using a magnetic field and the other using a radio wave. Inductive coupling is an example of transferring energy through a magnetic field. That is, an antenna generates a strong high frequency power signal and creates a magnetic field which penetrates the antenna coil of the RF tag. As a result, an electric current is induced, and the RF tag is driven by this current. On the other hand, backscatter coupling is an example of transferring energy by way of a radio wave. Thus, a portion of the power radiated by the antenna is reflected off the RF tag's antenna and is used for operating the RF tag.
In the case of passive RF tags, different levels of RF power are drawn to the RF tags, depending on the distance between an antenna and an RF tag. In other words, the smaller the distance between the antenna and the RF tag, the stronger the RF power drawn to the RF tag. As the RF power becomes greater than the typical RF tag's operational voltage range, 1.5V-5V, circuitry cannot be operated stably. The relation of the distance, rmax, between the antenna and the RF tag and power, PR, by which the RF tag can operate practically can be formulated as shown in Equation 1 below.
                              r          max                =                              λ                          4              ⁢              π                                ⁢                                                    η                ⁢                                                                  ⁢                                  G                  T                                ⁢                                  G                  R                                ⁢                                  P                  T                                                            P                R                                                                        [                  Equation          ⁢                                          ⁢          1                ]            where η is the efficiency of a rectifier, GT is a gain of the antenna, PR is the power by which the tag can operate, GR is the receiver gain, PT is the transmitted power and λ is the wave length of the power signal. According to Equation 1, the distance, rmax, between the antenna and the RF tag is inversely proportional to the power, PR, by which the RF tag can operate.
In order to avoid excessive voltage from flowing into the circuitry of the RF tag, an over-charge protection circuit, i.e., a limiter, is used.
FIG. 1 is a circuit diagram of an RF tag. As shown in FIG. 1, when RF power is supplied from an antenna to an RF tag, the RF tag converts the RF power into DC power using a diode D1 and a capacitor C1, and supplies the DC power to the limiter 10.
FIG. 2 is a circuit diagram of a limiter used in an RF tag according to a first embodiment of the related art. As depicted in the drawing, a limiter 10 comprises a plurality of serially connected semiconductor switching elements M0, M1, M2, M4, M5, and a semiconductor switching element M3 which is connected in parallel to the other switching elements M0, M1, M2, M4, M5. Here, each of the semiconductor switching elements shown in FIG. 2 is an n-channel MOSFET, and the parallel connected MOSFET M3 receives a gate voltage from one of the serially connected MOSFETs to allow a current to flow between its source and drain.
The limiter 10 makes sure that a maximum amount of current is supplied to a regulator 20 if the input voltage for operating the circuitry of the RF tag is below a reference voltage. On the other hand, the limiter 10 makes sure that that a minimum amount of current is supplied to the regulator 20 if an input voltage for operating the circuitry of the RF tag is above a reference voltage. For instance, suppose that a reference voltage in the circuit diagram of FIG. 2 is 5V, a leakage current is less than 100 nA when an input voltage is below the reference voltage, whereas a leakage current is greater than 100 nA when an input voltage is above the reference voltage. To satisfy these conditions, all current should flow into the regulator if a supply voltage from a rectifying element is less than 5V. In like manner, all current should be grounded rapidly if a supply voltage from a rectifying element is greater than 5V.
In practice, however, 0.45 μA of current has leaked from the reference voltage 5V (please refer to Table 1 below). That is to say, in the related art limiter, it is absolutely impossible for leakage current to increase rapidly even if an input voltage is above the reference voltage.
TABLE 1Input voltage (V)Leakage current2.50.053nA3.00.29nA4.013.15nA5.00.45μA5.52.0μA6.07.30μA
The reason why leakage current in the related art limiter does not rapidly increase when an input voltage is above the reference voltage can be explained by Equation 2 below, which expresses the properties of a current flowing in a MOSFET.
                              I          D                =                              1            2                    ⁢                      μ            n                    ⁢                      C            ox                    ⁢                      W            L                    ⁢                                    (                                                V                  GS                                -                                  V                  T                                            )                        2                    ⁢                      (                          1              +                              λ                ⁢                                                                  ⁢                                  V                  DS                                                      )                                              [                  Equation          ⁢                                          ⁢          2                ]            where, VGS is an input voltage between the gate and source of a MOSFET, and VT is a turn-on voltage supplied to the source for turning on the MOSFET. The other variables in equation 2 are well-known to persons of ordinary skill in the art. As can be seen in Equation 2, the properties of the current flowing in the MOSFET, ID, are proportional to a square of the voltage VGS between the gate and source. Given that the turn-on voltage VT of the MOSFET is constant, the greater the voltage VGS between the gate and source, the greater the increment amount of leakage current. However, above the reference voltage, the amount of leakage current is not rapidly increased.
FIG. 3 is a circuit diagram of a limiter used in a RF tag according to a second embodiment of the related art. As shown in FIG. 3, the limiter includes a plurality of serially connected n-channel MOSFETs MN0, MN1, MN2, MN4, MN5, and three pairs of n-channel MOSFETs (MN8, MN3), (MN9, MN6), (MN10, MN7) that are connected in parallel to the serially connected n-channel MOSFETs MN0, MN1, MN2, MN4, MN5. Here, a drain voltage of MN1 is input to the gate of MN10 to turn MN10 on. When MN10 is turned on, it supplies a current from a rectifying element to MN7. When MN9 is turned on, a source voltage is input to the gate of MN7 to turn MN7 on. However, MN1 should be turned on first for MN9 to be turned on. In other words, MN7 cannot be turned on until MN1 is turned on. Similarly, a source voltage of MN8 is input to the gate of MN6 that is serially connected to MN9, and MN6 is turned on. Since a source voltage of MN2 is input to the gate of MN8, MN3 should be turned on first for MN5 to be turned on. Meanwhile, a voltage from the rectifying element is input to the gate of MN3 via MN0, MN1, MN2 and MN4, so MN3, like MN0, is turned on when an input voltage exceeds the turn-on voltage of MN3.
In this limiter circuit, each of the serially connected MOSFETs MN0, MN1, MN2, MN4, MN5 is turned on according to an input voltage from the rectifying element, more specifically, is turned on in sequence until the input voltage reaches a predetermined reference voltage. And, among the three pairs of MOSFETs connected in parallel (MN8, MN3), (MN9, MN6), (MN10, MN7), MN10 is turned on when MN0 is turned on, and MN9 and MN7 are turned on almost simultaneously when MN1 is turned on. Moreover, MN8 and MN6 are turned on almost simultaneously when MN2 is turned on. A current is attenuated as it penetrates these MOSFETs being turned on.
Table 2 below lists leakage currents with respect to different input voltages in the limiter circuit of FIG. 3.
TABLE 2Input voltage (V)Leakage current2.510μA3.0112μA4.0897μA4.51.6mA5.02.4mA5.53.4mA6.04.5mA
As can be seen in Table 2, more than 100 μA of a leakage current is generated when an input voltage is above the reference voltage 5V, creating a current having excellent properties. But still, when an input voltage (3V for example) is below the reference voltage, although the leakage current must be less than 100 nA, the leakage current is 112 μA, which is too much. Consequently, the amount of a current supplied to the regulator 20 is rapidly reduced.
Therefore, there is a need to develop a limiter, in which a minimum amount of current flows into a regulator when an input voltage is below a reference voltage, whereas a maximum amount of current flows into a regulator when an input voltage is above a reference voltage. In this manner, it becomes possible to prevent an excessive voltage from flowing into the RF tag circuit when an input voltage is above the reference voltage, and to supply a sufficient amount of current to the regulator when an input voltage is below the reference voltage.